This invention relates to polyurethane films and more particularly relates to polyurethane films prepared from aqueous dispersions.
While ostensibly reactive with water, it has long been known that polyisocyanate polymers can be used to prepare aqueous polyurethane dispersions. Polyurethane dispersions are generally prepared by chain extending the reaction product of an organic diisocyanate or polyisocyanate and an organic compound having two or more active hydrogen atoms such as polyalkylene ether glycols, poly(alkylene ether-alkylene thioether) glycols, alkyd resins, polyesters and polyester amides, often using an organic solvent. The diisocyanate is used in stoichiometric excess so that the reaction product, also referred to as a polyurethane/urea/thiourea prepolymer, is isocyanate terminated. Examples of polyurethane prepolymer preparations are described in U.S. Pat. Nos. 3,178,310, 3,919,173, 4,442,259, 4,444,976, and 4,742,095, among others.
Polyurethane dispersions are reported as being useful for preparing such diverse materials as: coatings and bonds in U.S. Pat. No. 4,292,226; flexible solvent barriers in U.S. Pat. No. 4,431,763; adhesives in U.S. Pat. No. 4,433,095; and films in U.S. Pat. No. 4,501,852. Films, or rather the process of dipping to make a film, can be a part of the processes for making many articles. Examples of film applications include exam gloves, organ bags, condoms, ostomy bags, and the like. While it is known that such applications can be made with polyurethane dispersions, conventional polyurethane dispersions have sometimes been found to have insufficient physical or handling properties to make them a preferred material for such applications. Also, the use of a solvent can have adverse effects for some applications.
Polyurethanes are the reaction product of a polyalcohol and a polyisocyanate. Typically, the polyisocyanates used to prepare polyurethane dispersions have been aliphatic isocyanates such are disclosed in U.S. Pat. No. 5,494,960. Aromatic polyisocyanates such as toluene diisocyanate (TDI) and methylene diphenyldiisocyanate (MDI) as well as polymethylene polyphenylisocyanate are also known to be useful.
It is known to prepare polyurethane films by methods such as coagulation and casting. While these are commonly used, they are not without problems. For example, in coagulation processes, there is often a requirement for a step to remove coagulants from the films formed by means of washing or leaching. These steps often result in undesirable additional dewatering steps, undesirable waste streams and increased costs due to additional handling.
Accordingly, it would be desirable in the art of preparing polyurethane films from aqueous dispersions, to prepare such films which have physical and handling properties sufficient for their use in conventional film applications. It would be further desirable if such films could be prepared with dispersions which are, in turn, prepared in the absence of organic solvents. It would be even further desirable if such films could be prepared without undesirable extra steps.
In one aspect, the present invention is a process for preparing polyurethane films comprising electrodepositing the film onto a substrate from a polyurethane dispersion.
In another aspect, the present invention is a polyurethane film prepared with process which includes electrodepositing the film onto a substrate from a polyurethane dispersion.
One advantage of the electrodeposition process of the present invention is that there is no need for an additional step to remove coagulants from the films, as with conventional coagulation processes. Another advantage is that in using electrodeposition, the amount of water that must be removed during a dewatering step is minimized. A third advantage is that the resulting film demonstrates much greater uniformity in thickness as compare to films prepared by conventional coagulation techniques.
The present invention has applicability in, for example, gloves, condoms, medical bags, angioplasty balloons, medical bellows, face masks, blood pressure cuffs and the like. The present invention also has applicability in parts associated with drug delivery mechanisms, including catheters, medical tubing, gaskets and o-rings. Moreover, the present invention has applicability in many non-medical items, such as, for example, non-medical gloves, swim caps, tool handle grips, industrial caps and plugs, windshield wiper boots, toy balloons, toys, electrical parts, covers and gaskets.
The films of the present invention can be prepared from any polyurethane prepolymer dispersion which is sufficiently stable to be stored but not so stable that it cannot be electrodeposited. The dispersion can be prepared in any way which results in a dispersion which can be used to prepare a film having acceptable physical properties for the anticipated use of the film. The dispersions can be done by a batch process or by a continuous process. If done by a batch process, preferably, the dispersion in done by an inverse phase process wherein a small amount of water, including a small amount of anionic surfactant, is first added to a continuous prepolymer phase and mixed and then more water is added with mixing until the phase inverts.
When dispersions of the present invention are prepared by means of a continuous process, preferably they are prepared by means of a high internal phase ratio (HIPR) process. Such processes are known and are disclosed in, for Example, U.S. Pat. No. 5,539,021 to Pate, et al., and WO 98/41552 A1 to Jakubowski, et al. When prepared by either method, the resulting dispersion should have a particle size sufficient to make the dispersion stable. The dispersions of the present invention will have a particle size of from 0.9 to about 0.05, preferably from about 0.5 to about 0.07 and even more preferably, from about 0.4 to about 0.10 microns. Most preferably, the particle size of the dispersions of the present invention is about 0.15 microns.
The polyurethane dispersions of the present invention are prepared from a nonionic polyurethane prepolymer. The nonionic prepolymers of the present invention are prepared with an aliphatic or aromatic diisocyanate. Preferably, the diisocyanate is an aromatic diisocyanate selected from the group consisting of MDI, TDI and mixtures thereof. TDI can be generally used with any commonly available isomer distribution. The most commonly available TDI has an isomer distribution of 80 percent of the 2,4 isomer and 20 percent of the 2,6 isomer. For the purposes of the present invention, TDI with other isomer distributions can also be used, but often at significantly higher cost.
When MDI is used with the formulations of the present invention, it preferably has a P,Pxe2x80x2 isomer content of from about 99 percent to about 90 percent. Even more preferably, when MDI is used with the formulations of the present invention, it preferably has a P,Pxe2x80x2 isomer content of from about 98 to about 92 percent. Most preferably, when MDI is used with the formulations of the present invention, it preferably has a P,Pxe2x80x2 isomer content of about 94 percent. While MDI with such isomer distributions can be prepared by distillation during the MDI process, it can also be prepared by admixing commonly available products such as ISONATE 125M* and ISONATE 50OP*. (*ISONATE 125M and ISONATE 50OP are trade designations of The Dow Chemical Company.)
When mixtures of TDI and MDI are used to prepare the prepolymers of the present invention, they are admixed in a ratio of MDI to TDI of from about 99 percent MDI to about 80 percent MDI. More preferably, when mixtures of TDI and MDI are used to prepare the prepolymers of the present invention, they are admixed in a ratio of MDI to TDI of from about 98 percent MDI to about 90 percent MDI. Most preferably, when mixtures of TDI and MDI are used to prepare the prepolymers of the present invention, they are admixed in a ratio of MDI to TDI of about 96 percent MDI. Preferably the prepolymers of the present invention are prepared with MDI or mixtures of MDI and TDI. Even more preferably, the prepolymers of the present invention are prepared with MDI as the only aromatic diisocyanate.
In one embodiment of the present invention, the prepolymers of the present invention are prepared from a formulation that includes an active hydrogen containing material. In a preferred embodiment of the present invention, the active hydrogen containing material is a mixture of diols. One component of the diol mixture is a high molecular weight polyoxypropylene diol having an ethylene oxide capping of from 0 to 25 weight percent. The other component of the diol mixture is a low molecular weight diol. The polyether diols of the formulations of the present invention can be prepared by any method known to those of ordinary skill in the art of preparing polyether polyols to be useful for preparing such diols. Preferably, the polyether diols are prepared by the alkoxylation of a difunctional initiator in the presence of a basic catalyst. For example, a polyether useful with the present invention is a product resulting from a two step alkoxylation of ethylene glycol with first propylene oxide and then ethylene oxide, in the presence of KOH as a catalyst.
The high molecular weight polyether diol component of the diol mixture of the prepolymer formulations of present invention is a polyoxypropylene diol having an ethylene oxide capping of from 0 to 25 weight percent. Preferably, the molecular weight of this component is from about 1,000 to about 4,000, more preferably from about 1,200 to about 2,500, and most preferably from about 1,800 to about 2,200. As stated, the polyether diol is capped with from 0 to 25 percent ethylene oxide. Preferably, the high molecular weight diol is capped with from about 5 to about 25 percent ethylene oxide, and more preferably, from about 10 to about 15 percent ethylene oxide.
The low molecular weight diol component of some of the prepolymer formulations of the present invention can also be a product of alkoxylating a difunctional initiator. Preferably, this component is also a polyoxypropylene diol, but it can also be a mixed ethylene oxide propylene oxide polyol, as long as at least 75 weight percent of the alkoxides used, if present, is propylene oxide. Diols such as propylene glycol, diethylene glycol, dipropylene glycol, and the like, can also be used with the formulations of the present invention. The low molecular weight diol component of the prepolymer formulations, if present, has a molecular weight of from about 60 to about 750, preferably from about 62 to about 600, and most preferably, from about 125 to about 500.
The prepolymers of the present invention can be prepared in any way known to those of ordinary skill in the art of preparing polyurethane prepolymers to useful for preparing such prepolymers. Preferably the aromatic diisocyanate and polyether diol mixture are brought together and heated under reaction conditions sufficient to prepare a polyurethane prepolymer. The stoichiometry of the prepolymer formulations of the present invention is such that the diisocyanate is present in excess. Preferably, the prepolymers of the present invention have an isocyanate content (also known as % NCO) of from about 1 to about 9 weight percent, more preferably from about 2 to about 8 weight percent, and most preferably from about 3 to about 7 weight percent.
The prepolymers of the present invention are optionally extended with a difunctional amine chain extender when the active hydrogen containing material of the prepolymer formulation is a mixture of a low molecular weight diol and a high molecular weight polyether diol. The difunctional amine chain extender is not optional but required when the active hydrogen containing material of the prepolymer formulation is a high molecular weight polyether diol and does not include a low molecular weight diol. Preferably, the difunctional amine chain extender is present in the water used to make the dispersion. When used, the amine chain extender can be any isocyanate reactive diamine or amine having another isocyanate reactive group and a molecular weight of from about 60 to about 450, but is preferably selected from the group consisting of: an aminated polyether diols; piperazine, aminoethylethanolamine, ethanolamine, ethylenediamine and mixtures thereof. Preferably, the amine chain extender is dissolved in the water used to make the dispersion.
The prepolymers of the present invention are nonionic. There are no ionic groups incorporated in or attached to the backbones of the prepolymers used to prepare the films of the present invention. The anionic surfactant used to prepare the dispersions of the present invention is a external stabilizer and is not incorporated into the polymer backbones of the films of the present invention.
The prepolymers of the present invention are dispersed in water which contains a surfactant. Preferably the surfactant is an anionic surfactant. In the practice of preparing the dispersions of the present invention, the surfactant is preferably introduced into water prior to a prepolymer being dispersed therein, but it is not outside the scope of the present invention that the surfactant and prepolymer could be introduced into the water at the same time. Any anionic surfactant can be used with the present invention, but preferably the anionic surfactant is sodium dodecyl benzene sulfonate.
The dispersions of the present invention can have a solids level of from about 30 weight percent to about 60 weight percent. Films will not necessarily be prepared from dispersions having this level of solids. While the dispersions themselves will be stored and shipped at as high a solids content as possible to minimize storage volume and shipping costs, the dispersions can desirably be diluted prior to final use. The thickness of the film to be prepared will usually dictate what solids level is needed in the dispersion. When preparing films, the dispersions of the present invention can be at a weight percent solids of from 5 to about 60 percent, preferably from about 10 to about 40 percent, and, most preferably, from about 15 to about 25 weight percent when preparing examination gloves. Higher percent solids will generally result in a faster deposition rate using electrodeposition. For other applications, the film thickness and corresponding solids content of the dispersion used can vary.
For many film applications, it is necessary that the polymer used have physical properties similar to the more conventional materials used in those applications. For example, for an exam glove application, the films of the present invention can have a tensile set of less than 5%. One significant advantage of the films of the present invention over natural latex in applications such as exam gloves is that the films of the present invention have significantly lesser risk of inducing an allergic reaction in those wearing the gloves.
Another benefit of films of the present invention is that they can be prepared such that they are self-releasing. In the art of preparing exam gloves, this ability is also known as xe2x80x9cpowder freexe2x80x9d in reference to the fact that such gloves are occasionally prepared and sold with a layer of talcum powder, corn starch, or the like, to keep the polymer from adhering to itself, thereby making it easier to put on the gloves. The films of the present invention can be made self releasing by inclusion of a wax in the prepolymer formulation. Preferably the wax is carnauba wax and the like. It is preferable that the wax to be used be selected from those that are not likely to induce an allergic reaction in skin that comes in contact therewith. Therefore, food grade waxes are particularly preferred for this application. When used, the waxes are preferably included in the water used to disperse the prepolymer formulation at a concentration of from about 0.1 to about 2 weight percent.
In addition to the waxes already mentioned, other additives can be included in the films of the present invention. Any additive which is known to those of ordinary skill in the art of preparing films from dispersion to be useful can be used with the films of the present invention so long as their presence does not degrade the properties of the film so much that the film is no longer fit for its intended purposes. The additives can also be incorporated into the films in any way known to be useful including, but not limited to inclusion in the prepolymer formulation and inclusion in the water used to make the dispersion. For example titanium dioxide is useful for coloring films of the present invention. Other useful additives include calcium carbonate, silicon oxide, defoamers, biocides, carbon particles, and the like.
In using electrodeposition to form the polyurethane films of the present invention, the dispersion particles are forced to move toward an electrode by applying an electric field to the dispersion. The particles collect at the electrode and form a deposit on the electrode. The electrodeposition can be followed with an optional densification step if desired. In the present invention, a substrate is made to have a charge by attaching the substrate to a power supply. Preferably, the substrate is made to have a positive charge, that is, it is made to be an anode. The substrate can be made of any anodic material that is stable, for example, does not produce metal ions. Suitable anodic materials include, for example, glass, ceramic materials, graphite, various stainless steels, antimony-doped tin oxide, and plastics which are unmodified or which are made sufficiently conductive through additives.
Film thickness can be controlled by varying such parameters as the percent solids in the dispersion, the applied voltage and the duration of treatment.
Selection of voltages used for electrodeposition should be selected such that the resulting film is of the thickness and quality required for the particular application. Generally, higher voltages result in faster deposition and thus thicker films. Preferably, the applied voltage is at least about 0.5V versus a Ag/AgCl reference electrode, more preferably at least about 2.0V, and most preferably at least about 4.0V.
As for the mode of deposition, xe2x80x9cdead entryxe2x80x9d, xe2x80x9clive entryxe2x80x9d or xe2x80x9craised voltagexe2x80x9d may be used. Dead entry is where the substrate is first placed into a bath containing the polyurethane dispersion, followed by application of the voltage. Live entry is where the substrate is lowered into the dispersion bath with the voltage already applied. Raised voltage is where the substrate is lowered into the bath at low or no voltage, and then the applied voltage is increased at a set rate until a desired set point is reached.
The time of application will depend upon the thickness desired, the dispersion concentration and the applied voltage. Generally, the longer the voltage is applied, the thicker the resulting film. The desired thickness will depend upon the end use application.
The current can be measured as the film is deposited on the substrate. A decrease in current indicates that a film is indeed being formed, thereby creating a resistance and lowering the current. The electrodeposition step can be followed by more conventional coating techniques, such as, for example, dip coating, if desired.